1. Field of the Invention
The present invention relates to a composite high-frequency component that includes a high-frequency switch, and, in particular, to a composite high-frequency component that switches between transmission/reception signals with a low control voltage.
2. Description of the Related Art
A known composite high-frequency component transmits/receives transmission/reception signals of a plurality of communication systems in different frequency bands through an antenna and includes a high-frequency switch switching the transmission/reception signals to a transmission signal and a reception signal (see, for example, Japanese Unexamined Patent Application Publication No. 2000-165274).
The high-frequency switch includes a plurality of diodes to output a transmission signal to the antenna and to input a reception signal from the antenna into a reception portion.
The configuration of a composite high-frequency component shown in Japanese Unexamined Patent Application Publication No. 2000-165274 is described below and shown in FIG. 1.
This composite high-frequency component supports GSM1800 (DCS) in the 1800 MHz range, GSM1900 (PCS) in the 1900 MHz range, GSM850 in the 850 MHz range, and GSM900 (EGSM) in the 900 MHz range.
In FIG. 1, a diplexer (multiplexer/demultiplexer) 102 multiplexes or demultiplexes a transmission/reception signal of the GSM850/GSM900 system and a transmission/reception signal of the GSM1800/GSM1900 system. A transmission/reception selection switch 103 switches between a transmission signal and a reception signal of the GSM850/GSM900 system. Similarly, a transmission/reception selection switch 104 switches between a transmission signal and a reception signal of the GSM1800/GSM1900 system. A filter 105 allows the fundamental of a transmission signal of the GSM850/GSM900 system to pass therethrough and attenuates the harmonic. Similarly, a filter 106 allows the fundamental of a transmission signal of the GSM1800/GSM1900 system to pass therethrough and attenuates the harmonic.
In the transmission/reception selection switch 103, a diode GD1 and an inductor GSL1 are disposed in a transmission line though which a transmission signal of the GSM850/900 system is transmitted, and the diode GD1 is disposed in series with the transmission line for the transmission signal. A strip line GSL2, a diode GD2, and a capacitor GC5 are disposed in a transmission line through which a reception signal of the GSM850/900 system is transmitted, and the diode GD2 is disposed in shunt with the transmission line for the reception signal. The two diodes GD1 and GD2 are connected in series such that, when a positive voltage is applied from a terminal VcG for a transmission/reception selection control signal, a direct current passes through the diodes GD1 and GD2.
Similarly, in the transmission/reception selection switch 104, a diode DD1 and an inductor DPSL1 are disposed in a transmission line through which a transmission signal of the GSM1800/1900 system is transmitted, and the diode DD1 is disposed in series with the transmission line for the transmission signal. A strip line DSL2, a diode DD2, and a capacitor DC5 are disposed in a transmission line through which a reception signal of the GSM1800/1900 system is transmitted, and the diode DD2 is disposed in shunt with the transmission line for the reception signal. The two diodes DD1 and DD2 are connected in series such that, when a positive voltage is applied from a terminal VcD for a transmission/reception selection control signal, a direct current passes through the diodes DD1 and DD2.
The above-described composite high-frequency component is used in a communication device in a mobile communication system, such as a mobile telephone terminal, and the driving voltage therefor has been continuously reduced with a recent demand for power saving. However, when a voltage of a control signal in a known transmission/reception selection switch (high-frequency switch) illustrated in FIG. 1 is reduced, a voltage applied between the anode and cathode of each of the two diodes (for example, the diodes GD1 and GD2 in the transmission/reception selection switch 103 for the GSM850/900 system illustrated in FIG. 1) approaches the PN junction potential of the diode. Under this condition, the diode is incompletely conducting, so a phenomenon occurs in which ON resistance increases or a voltage between both ends of the diode varies in accordance with a voltage of a transmission signal to be transmitted/cut off. As a result, a problem arises in which an insertion loss (IL) and harmonic distortion increase. Consequently, there is a limit to the amount that the control voltage can be reduced.